[Haskell-beginners] When a return type constrained by class

[Haisheng Wu]

Hi there,
 Do you have any idea why the following code is not valid?

class Guess g where
 makeGuess :: g -> String

instance Guess GuessLetter where
 makeGuess = ...

instance Guess GuessWord where
 makeGuess = ...

*-- | This function is nod valid *
nextGuess :: Guess g => String -> g
nextGuess = ...


Thank you.

-Haisheng
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